Established in 1908, Natural Bridges National Monument is Utah's oldest national monument. It protects three of the largest known natural bridges in the world, along with Ancestral Puebloan sites. Natural bridges are made by flowing water: the monument's three were cut by streams in White Canyon and Armstrong Canyon, through the Permian Cedar Mesa Sandstone.
In 1904, articles in Century Magazine and National Geographic brought the bridges to national attention; reaching them then took days on horseback. The first written description of Sipapu Bridge measured it this way:
This bridge would overspan the Capitol at Washington, and clear the top of the dome by 51 feet; and if the loftiest tree in the Calaveras grove of giant sequoia in California stood in the bottom of the canyon, its topmost bough would lack 32 feet of reaching the underside of the arch.
In 1909 the bridges were given Hopi names, honoring the region's Indigenous heritage.
The three bridges
| Sipapu | Kachina | Owachomo | |
|---|---|---|---|
| Name | "the place of emergence" | — | "rock mound," for the small dome above one end |
| Height | 220 ft (67 m) | 210 ft (64 m) | 106 ft (32 m) |
| Span | 268 ft (82 m) — the largest in the monument | 204 ft (62 m) | 180 ft (55 m) |
| Thickness of span | 53 ft (16 m) | 93 ft (28 m) — stout | 9 ft (3 m) — very thin |
| How it formed | a meander in White Canyon was cut off and the stream took a shorter course beneath the bridge, abandoning the old loop | near the meeting of White and Armstrong canyons, when the stream cut a new course to the confluence, leaving an abandoned meander | near the meeting of Armstrong Canyon and a tributary, which twice cut new routes into Armstrong Canyon — so the bridge no longer spans the main channel |
| Relative age | middle-aged, with a well-shaped opening | youngest | oldest: thin span, large opening, cut off from the main channel |
Their ages are judged from their size, shape, openings and thickness. Exact dates are hard to get for landforms like these, but they are geologically very young — perhaps tens of thousands of years.
How natural bridges form
Natural bridges form where streams cut through narrow canyon walls. Rock spans like bridges and arches grow through decay, weathering and erosion, with big rockfalls playing a large part — in 1992, about 4,000 tons of rock fell at Kachina Bridge. Groundwater sapping probably helped too, given how porous and permeable the Cedar Mesa Sandstone is.
Most of Utah's arches and bridges are in massive sandstones like this one: uniform grains, few bedding planes, and the strength to stand in vertical and overhanging cliffs. Sipapu and Kachina are among the largest natural spans in the world; the very largest known, in China, are karst features, as is the famous bridge at Virginia's Natural Bridge State Park, a National Park Service affiliated area. In Utah, only Rainbow Bridge and Morning Glory Bridge near Moab have longer spans than Sipapu.
The setting: Colorado Plateau
The monument lies in the heart of the Colorado Plateau, one of 25 physiographic provinces in the lower 48 states: high, with thick crust and little deformation, about 130,000 square miles (337,000 km²) across Utah, Colorado, New Mexico and Arizona — home to 30 units of the National Park System, the country's greatest concentration. A dry climate and the erosive power of the Colorado River, about 29 miles (47 km) west of the monument, and its tributaries have laid the rock layers bare.
Natural Bridges sits in the plateau's Canyon Lands section, gently tilted layers carved into canyons, on the Monument Uplift — a great fold that runs from Monument Valley north to about the meeting of the Green and Colorado rivers in Canyonlands. Its steep eastern edge is Comb Ridge, about 18 miles (29 km) east. At the uplift's center, the erosion-resistant Cedar Mesa Sandstone holds up Cedar Mesa, a plateau about 6,500 feet (2,000 m) high and the geologic centerpiece of the surrounding Bears Ears National Monument, managed by the Bureau of Land Management. White Canyon is named for the pale sandstone in its walls.
The rocks
The monument's rocks were laid down between about 310 and 205 million years ago, while the Ancestral Rocky Mountains to the northeast were wearing away.
| Rock unit | Age | What it is |
|---|---|---|
| Cedar Mesa Sandstone (Cutler Group) | Permian | the bedrock across almost the whole monument: very fine white sandstone with steep crossbeds — wind-blown dunes of a vast sand sea (erg) — with rare thin red mudstone; about 800 ft (250 m) thick here |
| Organ Rock Formation (Cutler Group) | Permian | red sandstone, siltstone and mudstone from rivers and local dune fields, about 300 ft (100 m) thick; exposed in two small areas |
| Moenkopi Formation | Triassic, about 240 million years ago | thin brick-red beds with abundant ripple marks, from tidal flats and river floodplains in a hot, dry climate |
| Chinle Formation | Late Triassic, about 230–205 million years ago | the Shinarump Conglomerate at its base, then colorful mudstones and ledgy sandstones from monsoon-fed rivers |
Uplift and carving
These rocks formed at or near sea level; now they stand as high as 6,500 feet, and 4,000 to 6,000 feet of younger rock that once buried them has been eroded away.
| When | What happened |
|---|---|
| 70–40 million years ago | the Laramide Orogeny, driven by a thick oceanic plateau subducting at a shallow angle, raised the Rockies and the Colorado Plateau and formed the Monument Uplift and folds like the Waterpocket Fold |
| 35–25 million years ago | the mid-Cenozoic uplift; shallow intrusions formed the La Sal, Abajo and Henry mountains around the park |
| Last 10 million years | further uplift from deep mantle processes |
| Last 5–6 million years | the upper Colorado River connects to the Gulf of California, and canyon cutting begins |
| Last 2–3 million years or so | rapid erosion carves the central plateau's landscape |
Across the plateau, erosion has stripped away 5,000 to 13,000 feet (1,500 to 4,000 m) of rock, most along the Colorado itself. The river's cutting drives erosion up tributaries like White and Armstrong canyons, as cliffs retreat by rockfall and sapping. In the Canyonlands region just north, erosion over the past several hundred thousand years has averaged 1,300 feet (400 m) per million years or more — among the fastest on the plateau.
A pool in a canyon. NPS
More than bridges
- Biological soil crusts — cyanobacteria, mosses, lichens, algae, fungi and bacteria — are among the best developed in the world here. They fix nitrogen, feed the soil and hold it against erosion, but a footprint or tire track can destroy them, and recovery takes decades, if it happens at all. Stay on roads and trails; off trail, walk on bare slickrock or in dry washes.
- Springs, seeps and hanging gardens are rare, reliable water sources, formed where a less permeable layer turns groundwater sideways out of the porous sandstone.
- Desert varnish — clay, iron and manganese skins, silica glaze — streaks the pale cliffs brown and black, often where water runs down them.
- Fossils are scarce in the Cedar Mesa Sandstone: root traces; bones and dung of an extinct mountain goat in a rock shelter's Quaternary sediments; and plant microfossils from a Pleistocene packrat midden.
More: the NPS Geologic Resources Inventory maps and report for the monument, and the park's geology page.
Sources
Based on "NPS Geodiversity Atlas — Natural Bridges National Monument, Utah," National Park Service; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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